1. Field of the Invention
Copper exerts a large influence on the characteristics of silicon wafers. The present invention relates to a method for assaying copper in silicon wafers which is capable of the quantitative, accurate and highly sensitive detection of copper in silicon wafers. In particular, the present invention relates to a technique suitable for detecting low concentrations of copper below 1011 atoms/cm2 that are present in high-concentration boron-doped silicon substrates containing at least 3×1018 atoms/cm3 of boron.
This application claims priority from Japanese Patent Application No. 2006-45031 filed on Feb. 22, 2006, the content of which is incorporated herein by reference.
2. Background Art
In silicon wafers employed as substrates for semiconductor devices and the like, higher circuit integration and device scaling have made it imperative to lower the levels of metallic impurities (e.g., iron, nickel, chromium, copper) in the silicon wafers that seriously degrade device performance. Contamination by metallic impurities in silicon wafer fabrication is thought to arise in the polishing operation on p-type silicon wafers; copper in the polishing slurry diffuses into the bulk material of the wafer, leading to copper contamination. Among the contaminating metals that emerge, copper has a very rapid diffusion rate and easily diffuses into the interior of the silicon substrate. Because such diffused copper degrades the device characteristics (e.g., electrical properties), it is important to reduce the level of copper and control the process.
Recent improvements in contamination control technology, including the cleaning of silicon substrates, has brought the concentration of silicon substrate-contaminating metals down to about 1011 atoms/cm2.
In carrying out such measures against copper contamination in p-type silicon wafers, it is important for the copper diffused in the bulk material of the silicon wafer to be quantitatively determined to a high precision and accuracy in each step. The techniques used for quantifying the copper diffused in the bulk material of a silicon wafer have hitherto been primarily atomic absorption spectroscopy (AAS) and secondary ion mass spectroscopy (SIMS). AAS in particular is capable of high-sensitivity analysis. AAS is employed using what is known as the total dissolution method in which a portion of a polished silicon wafer is gas-etched with a mixed acid of hydrofluoric acid and nitric acid or a mixed acid of hydrofluoric acid, nitric acid and sulfuric acid, and the decomposition residues after the silicon wafer has dissolved are analyzed.
Such methods have a number of drawbacks. Measurement is very troublesome to carry out, and additional contamination sometimes occurs during pre-treatment prior to measurement. Moreover, because all such methods involve destroying the substrate, re-use of the substrate is impossible.
In this connection, the applicant earlier proposed, as a way of non-destructively analyzing semiconductor substrates, a method for detecting the copper concentration at the interior of a semiconductor substrate (Patent Document 1). This method is exemplified by the LTD (Low temperature diffusion) method which involves heating the silicon substrate to a temperature of 600° C. or less so as to induce copper present within the silicon substrate to diffuse out and collect at the front and rear sides of the substrate, and analyzing the front and rear sides by AAS or total reflection fluorescent X-ray analysis (TXRF). In this method, when the silicon substrate has a p-type conductivity, heating at 500° C. in an open air atmosphere for 15 minutes will result in sufficient diffusion of the copper.
In another method, a polysilicon layer is formed on the polished silicon wafer, the copper in the bulk material is diffused into this polysilicon layer by heat treatment, and the copper within the polysilicon layer to which copper has been diffused is then analyzed (Patent Documents 2 and 3).
Hence, three methods are commonly known for quantitatively determining copper that has diffused into the bulk material in polishing operations: a method in which polysilicon (Poly-Si) is applied to the polished silicon wafer, heat treatment is applied to diffuse copper within the bulk material into the Poly-Si layer, and the copper within the Poly-Si layer is analyzed; an LTD method in which the polished silicon wafer is heat-treated on a hot plate, outwardly diffusing the copper to the surface, and the surface copper is analyzed; and a total dissolution method in which part of the polished silicon wafer is subjected to gas etching with a mixture of hydrofluoric acid and nitric acid or with a mixture of hydrofluoric acid, nitric acid and sulfuric acid, and the decomposition residues after dissolution of the silicon wafer are analyzed.
However, the above prior-art methods for quantifying the copper diffused in the bulk material of a silicon wafer pose a number of challenges with regard to the precision and accuracy of detection. For example, in a method that involves forming a polysilicon layer on the polished silicon wafer and thermally diffusing copper into this polysilicon layer by heat treatment, and then analyzing, polysilicon is applied to the silicon wafer after the polishing operation, leading to the formation of a polysilicon layer on both sides of the silicon wafer. Hence, the copper that has diffused into the bulk material because of polishing disperses out to the polysilicon layers on either side of the wafer during heat treatment, rendering high-precision analysis impossible.
Also, when polysilicon is applied to the polished silicon wafer, the amount of copper that was already diffused in the silicon wafer prior to polishing becomes unclear. Because quantitative determination of the copper is possible only after polishing, the amount of copper that diffused into the silicon wafer during the polishing step cannot be determined.
In the LTD method which involves heat-treating the polished silicon wafer on a hot plate and analyzing the copper that has outwardly diffused to the surface, when the wafer is heat-treated on a hot plate, sometimes 100% of the copper in high boron concentration wafers (p+, p++) does not diffuse outward to the wafer surface, thus making it difficult to accurately and rapidly determine the total copper in the wafer. Moreover, in all p-type silicon wafers, when the wafer is heat treated on a hot plate, the copper does not diffuse outward to the front and rear sides of the wafer and disperses, making high-sensitivity analysis impossible.
In a total dissolution method in which a portion of a polished silicon wafer is dissolved with a mixed acid and the dissolution residues are analyzed, to carry out quantitative analysis of the metallic impurities in the residues with an atomic absorption spectrophotometer or an inductively coupled plasma mass spectrometer, it is necessary to remove the large amount of silicon present in the recovered solution. Removing such silicon by sublimation involves dissolving the silicon with a mixed acid such as hydrofluoric acid, nitric acid and sulfuric acid and carrying out concentration. However, because such an approach requires the use of a large amount of chemicals, metallic impurities present in the chemicals are also included during quantitative analysis. When concentration is carried out over an extended period of time, there is even a possibility that impurities in the atmospheric air will be taken up, making an accurate determination difficult to carry out. Furthermore, because the silicon wafer after it has been polished is completely dissolved, the amount of copper diffused within the silicon wafer before polishing is unclear; it is possible only to determine the amount of copper in the wafer after it has been polished.
It is therefore an object of the present invention to provide a method for assaying the copper within a silicon wafer, which method involves no complicated operations and can accurately detect to a high sensitivity the concentration of copper in a silicon wafer.
Patent Document 1: Japanese Patent Application, First Publication No. H09-64133
Patent Document 2: Japanese Patent Application, First Publication No. H10-223713
Patent Document 3: Japanese Patent Application, First Publication No. 2004-335955